** FILE ** In this Oct. 25, 1992 file photo, sailors are seen aboard the HMS Vanguard, in Holy Loch, Scotland. Nuclear-armed submarines from Britain and France collided deep under the Atlantic Ocean earlier this month, causing damage to both vessels but releasing no radioactivity, a British official said Monday, Feb. 16, 2009. The HMS Vanguard, Britain's first Trident class nuclear-armed submarine, and the French Le Triomphant submarine, which was also carrying nuclear missiles, both suffered minor damage. (AP Photo/PA, Chris Bacon, File) ** UNITED KINGDOM OUT, NO SALES, NO ARCHIVE **

In a freak accident, two submarines carrying nuclear weapons, one French and the other British, collided while submerged on operational patrols in the Atlantic earlier this month, the countries' defense ministries said Monday.

Both vessels returned damaged but otherwise safe to their home ports, with the 250 crew members abroad uninjured and with "no compromise to nuclear safety," the defense ministries said in terse statements that appeared to have been agreed between the nations. The reference appeared to cover the nuclear reactors that power the submarines and the 16 ballistic missiles carrying nuclear warheads that the British and French vessels each routinely carry on patrols.

But military experts said the episode raises troubling questions about the safety of ballistic-missile submarines patrolling the oceans while hiding their whereabouts even from NATO allies. They said that agreements on "water-space management," requiring NATO nations to advise each other of the whereabouts of submerged submarines, do not include vessels carrying ballistic missiles with nuclear warheads.

The collision spurred a fresh outcry from groups in Britain and France that have demanded that the nations scrap their nuclear arsenals, with representatives saying that only chance prevented a more serious impact that could have sunk both vessels, along with their missiles.

The collision "could have released vast amounts of radiation and scattered scores of nuclear warheads across the seabeds," said Kate Hudson, chairwoman of the Campaign for Nuclear Disarmament, a long-established protest group in Britain.

The collision of the vessels on the night of Feb. 3, at a location neither nation disclosed, was described by military experts in London and Paris as a million-to-one occurrence, given the expanse of the oceans and the low number of submarines carrying ballistic missiles on patrol at any time from nations with such vessels. Those nations include the United States, Russia and China, as well as Britain and France.

Just as startling, the experts said, was that the French Defense Ministry appeared not to have known in the immediate aftermath that its submarine, Le Triomphant, had struck the British submarine, Vanguard. On Feb. 6, the ministry released a statement in Paris saying the French vessel had "collided with an immersed object," which it described as probably a drifting cargo container, and that the submarine's sonar dome, located in its nose and crucial to its ability to track other vessels, had been seriously damaged.

Official confirmation of the collision came only after a report of the episode appeared Monday in the Sun, a British tabloid newspaper. French officials said Monday they only realized that Le Triomphant had struck the British vessel after sending inquiries to other navies about the deep-sea impact - an admission that appeared to underline the extreme secrecy NATO allies impose on the whereabouts of their missile-carrying submarines.

The Vanguard, which is 492 feet long, was towed back to its home port at Faslane on the Firth of Clyde, near Glasgow, Scotland, with "very visible dents and scrapes," according to the BBC. The similarly-sized French submarine took three days after the impact to return to its home port, at L'Ile Longue near Brest, according to reports in the French news media.

Military experts said the only previously known collision between two nuclear submarines occurred in 1992, when an American submarine, the Baton Rouge, was struck by a surfacing Russian submarine in the Barents Sea.